As states experiment with increasing and lowering speed limits on rural highways, questions still remain as to what effects those changes might have on safety. At this year’s annual meeting of the Transportation Research Board, researchers trying to answer some of those questions found that slower is safer.

In one study, researchers from the Loughborough University in the U.K. analyzed more than 10,000 crashes on England’s highways in the year 2012 and concluded that each one percent increase in traffic speed corresponds with an equivalent increase in the number of fatal crashes. Higher speeds also increase the likelihood of crashes involving serious and minor injuries, but the effects are smaller (0.7 and 0.5 percent, respectively).

Their work improves upon earlier studies, in which crashes along road segments up to several miles long were typically grouped together even though the traffic conditions and road geometry were often quite different. In this study, researchers knew the precise time of a crash (within a 15-minute interval) and the geographic location, so they were able to test for the effects of traffic conditions leading up to each crash and the specific road geometry.

Research based in the U.S., looking at the aggregate effects of speed limits on road safety, seems to corroborate the U.K. work. Researchers from Wayne State University in Michigan studied state-level fatality rates on rural highways between 1999 and 2011. Their main interest was in the effect of the maximum posted speed limit in each state, but they also considered the effects of traffic volumes, highway mileage, the number of registered vehicles and licensed drivers, total population, seat belt laws, climate, and gas prices.

They found that fatality rates are lowest in states with maximum speed limits between 60 and 65 mph. States with 70 mph speed limits experience 22 percent more fatal crashes. States with speed limits of 75 mph or higher experience 52 to 125 percent more fatal crashes, even though fatality rates have dropped the most in those states since 1999. Moreover, the number of fatal crashes increases much more rapidly with the number of miles traveled in states with speed limits above 65 mph.